Cesium for Unity only includes binaries for Windows (Intel), macOS (both Intel and Apple), Android (Intel and ARM), and iOS. Only 64-bit platforms are supported, there is no support for any 32-bit platform.
If your Docker image isn’t one of the supported platforms, then that is the problem.
If you’re trying to build Cesium for Unity yourself from source rather than using the version that includes pre-built native code, you’ll need to follow the Developer Setup instructions.